This invention relates to electrical circuits for processing sensed audio signals, and more particularly to hearing aid circuits for selectively suppressing noise in low and medium frequency ranges.
With increasingly sophisticated tools at their disposal, designers of hearing aids are addressing one of the more difficult challenges in the design of quality hearing aids; the rejection of environmental noise. The power spectral energy of background noise is predominantly in the low audible frequency ranges, and tends to mask out relatively weaker high frequency components of speech. People with cochlear hearing impairments typically have trouble discriminating between speech and background noise, probably due to their greater susceptibility to masking as compared to persons with normal hearing. The problem is particularly acute in factories, at large social gatherings or other high background noise environments.
Generally, techniques to counter this problem involve improving the signal to noise ratio, emphasizing high frequency signals. The hearing aid microphone can be located where head diffraction is most favorable, directional microphones may be employed, or the user fit with binaural hearing aids.
Signal processing techniques, particularly high-pass filtering, are frequently utilized. U.S. Pat. No. 4,490,585, to Tanaka shows a low frequency detecting circuit, the output of which is provided to an automatic high-pass filter circuit in order to change the cut-off frequency of the automatic filter circuit in accordance with the level of the detecting circuit output. U.S. Pat. No. 4,119,814 to Harless granted Oct. 10, 1978, describes a continuously controlled or switched transistor circuit for increasing the cut-off frequency, above which there is provided a twelve dB/octave rise in response.
Another common related approach is to divide the hearing aid microphone output into separate frequency bands. In U.S. Pat. No. 4,596,902 to Gilman granted June 24, 1986, a processor compares actual sound pressure levels with desired levels in each band, and controls amplifiers associated with particular bands to obtain the desired output levels.
While such systems can be satisfactory in their processing of signals, they frequently utilize complex circuitry that is costly, and requires a hearing aid sufficiently large to accommodate the circuitry, in direct conflict with the customer's desire for a hearing aid as small and unobtrusive as possible. Furthermore, hearing aids which employ only low frequency sound to control a variable cut-off frequency cannot respond to excessive background noise in medium frequency ranges. And, when additional amplification in the form of automatic gain control is required, the accompanying attack and recovery artifacts of automatic gain control circuitry interfere with the processed audible signal.
Therefore, it is an object of the present invention to provide hearing aid circuitry which selectively suppresses low and medium frequency noise components, responsive to substantially the entire frequency spectrum of sound.
Another object is to provide a hearing aid with simple signal processing circuitry which can be implemented as a single monolithic integrated circuit chip.
A further object of the invention is to provide a signal processing circuit utilizing adaptive high-pass filtering without the need for automatic gain control circuitry.
Yet another object of the invention is to provide a process and apparatus for selectively enhancing part of an audio signal, utilizing adaptive high-pass filtering controlled in response to the broadband audio signal.